Other links have shown that disbelief in God is at a high, that would seem to corroborate that the trend isn't solely due to different ways of counting membership; though that is almost certainly a factor in the decline of church membership; we are also seeing lower attendance. IMHO, mega-churches aren't a sign of growth, they're a sign of smaller churches having more trouble paying their bills and being forced to consolidate.
I'm not anti-Christian. I'm anti-stupid.